Rechargeable lithium-ion and lithium-polymer batteries are used widely. Big batteries are used in electric vehicles or hybrid vehicles, storage batteries for solar or wind mill. Small batteries are used in notebook computers, mobile phones and other portable devices such as MP3 player, GPS receivers, and bluetooth earphones. These batteries have electrodes of lithium intercalating materials which is safe as there are no lithium in the form of metal exists in the batteries. The energy density is also much higher than other rechargeable technology like Ni—MH or Ni—Cd batteries. The trend to use rechargeable lithium based batteries to replace the primary batteries or other chemistry rechargeable batteries, in many aspects, is clear and present.
For portable electronic devices, batteries with higher volumetric energy density are always preferred. Such devices do not have space to contain a big battery. Soft pouch or lithium polymer prismatic battery in bluetooth earphones are one of the typical application. One example as U.S. Pat. No. 6,451,472 described, the spiral wound electrodes are pressed into prismatic shape and put into pouch, then using heat to melt the rim of plastic coated on aluminum foil core layer to complete the seal. Such batteries could be as small as 0.3 cubic centimeters at size of 4 mm×8 mm×10 mm, to its typical size of bluetooth earphone battery of about 1 cubic centimeter at size of 4 mm×11 mm×22 mm. Said battery has at least 3 sealing rims, two side rims, and one top rim, where the terminals come through. At this small volume, these sealing rims usually at a thickness of 0.15 mm to 0.50 mm, occupy quite a significant volume and leave less space for the active materials or the electrodes inside the battery pouch. So this kind of soft pouch battery is not the ideal candidate for very small electronic devices like hearing aid.
Button cell design for lithium based rechargeable batteries could be a better solution for very small electronic devices, while it has to have the least non-active volume for its casing and sealing configuration. In lithium based batteries, different sealing methods are used including glass-to-metal seal, laser welding or plasma welding, one example as described in U.S. Pat. No. 6,984,468. These sealing methods using thick casing design occupying significant volume so not suitable for lithium based button cell batteries, which commonly sealed by mechanical crimping of the outer casing pressed on the gasket and inner casing. Different from zinc air batteries, the inner casing side wall are folded double side walls for these lithium based button cells, as described in U.S. Pat. No. 5,629,107, U.S. Pat. No. 6,713,215 and U.S. Pat. No. 7,276,092.
Such folded double side walls give reliable seal of the battery because it has the highest closing pressure between the ending rim of the folded double side wall and the insulation gasket. The rim of the folded side wall is actually projected into the gasket by closing crimping force. Obviously the disadvantage is that the inner volume for active materials is reduced by these double walls.
Alternative button cell design uses single side wall, examples for hearing aid primary zinc air battery which being used for 3 decades very successfully, as U.S. Pat. Nos. 5,582,930 and 6,248,476. These zinc air button cells use every efforts to reduce its non-active casing space, by using thinner casing materials, by using single side wall anode top casing, to have high volumetric energy density. Such battery is also sealed by mechanical crimping of the battery outer casing to press the gasket firmly on the inner casing.
In U.S. Pat. No. 6,265,100 a rechargeable lithium button cell is described. It has an outer casing, but the inner casing or inner top does not have a side wall at all, it is just a flat round plate. How to seal such a battery was not discussed. No mechanical crimping means is available on the battery housing, no glass-to-metal seal or welding being applied. The inner space could be maximized by such design but how to realize the closing to seal the battery is quite questioned.
The volumetric energy density of rechargeable lithium based button cell is not only affected by the casing design and the sealing means design, but also affected by the inner electrodes arrangement. Some prior arts are:    a. Two solid plates of electrode, as U.S. Pat. No. 6,713,215 and U.S. Pat. No. 7,276,092, not with thin film type of electrodes. Such design is not suitable for lithium-ion rechargeable button cells.    b. Thin film type of electrodes piece by piece in stack, all layers parallel to battery top or bottom, one commercial product with brand name PowerDisc detail PD2032 at www.powercellkorea.com. Route JD company, Korea. Another design example is as U.S. Pat. No. 6,265,100. Such design uses multi-layers round thin film electrodes stacked with separators. The stack is round and occupies most space of the inner volume of the battery. However it has to leave a ring-like free space to let the current collectors of each layer folded and linked to gather together and connecting to the casings.    c. Thin film type of electrodes spiral wound to be a roll and pressed to prismatic shape, like the prismatic soft pouch battery design, roll axis is parallel to battery top or bottom. This design is simple than stack, but as the pressed roll is in rectangular shape, to put it into round battery casing wastes four arcing space between the electrodes and battery casing, hence the energy density is low. Such design is widely used in China made lithium ion rechargeable button cell batteries available in the market.    d. Thin film electrodes spiral wound roll, in its cylindrical shape, axis is vertical to battery top or bottom, just like the traditional cylindrical lithium-ion batteries one example as U.S. Pat. No. 5,427,874. Similar design was also mentioned in U.S. Pat. No. 6,265,100. No actual lithium-ion button cell is made in this way yet.
This last spiral wound electrodes arrangement with axis vertically to the button cell flat top or bottom, has high utility of the inner volume of the cylindrical cell. The prior arts electrodes width is constant at its full length and after being wound, the roll height is constant. This is not an issue in cylindrical lithium ion batteries as they are quite tall and the closing means is at the very top of the battery and does not interfere with the electrodes, structure example as U.S. Pat. No. 5,427,874. But for button cell, the space is very limited and the closing means is different from cylindrical cell. The button cell cap is only flat in the middle area, as it has to have a closing rim or closing shoulder wherein the height is lower than the flat top area, the electrodes figuration also need to be carefully designed to adapt to this height change. So the electrodes with constant width are not suitable or could not fully utilize the inner space of the battery. This was not mentioned in any prior arts.
Such vertical wound electrodes also need to be connected to the battery casing in a reliable, low impedance, space-saving, and easy-to-assembly way, which is another challenge to have a high performance button cell. The best solution for reliable connection is by welding, to connect the electrodes current collectors to the battery casings. This is similar like the cylindrical cell design but very difficult to be implemented in the same way into the very small button cell. The other solution could be by a face-to-face contact, to have the electrode roll outer surface to contact the inner surface of battery casing, but such design does not have stable impedance in battery cycles, because the electrodes will shrink in discharge and expand in charge, so the contact tightness varies with such shrink/expand cycle, and the battery performance is not stable.
Another consideration for high performance rechargeable lithium-ion button cell is the cycle life, it is preferred to be as long as possible, at least several hundreds cycles. The lithium-ion batteries are charged at 4.2V or even higher, at this potential, the cathode current collector and cathode casing metal wherein contacting the electrolyte are easy to be oxidized. The electrons will be lost from these metals then the metals become metal ions and resolve into electrolyte. These ions are impurities to the active materials and hence the battery may fail or has low cycle life. And so, aluminum or aluminum coated material is commonly used as the cathode current collector and cathode casing, because aluminum is very resistant to such oxidizing potential due to its surface oxidation film which protect it from being further oxidized.
In the lithium button cell casing design, stainless steel is commonly used, which also has some level of resistant to the oxidizing potential during charge, but not as good as aluminum. The lithium ion button cell batteries in the market with stainless steel casings usually have the cycle life of 300 cycles at 80% remaining capacity. This is good, but not good enough. Aluminum casing is not preferred because they are quite soft and it would be too thick occupying too much space for button cell if they having enough strength. Clad metal with stainless steel and aluminum could be used to combine the benefits of strength and anti-corrosion of each metal. Coating of aluminum or carbon on the cathode casing could be another solution as mentioned in U.S. Pat. No. 7,348,102. By other words, if the button cell is required to have long cycle life, cathode current collector and casing have to be carefully chosen using anti-corrosion means and/or materials.
Therefore, it is the object of the present invention to provide a reliable and minimized sealing means, for the rechargeable lithium-ion button cell, to leave maximum space for inner materials, to make the battery with high energy density.
It is another object of the present invention to provide novel electrodes configuration and arrangement in the battery housing, to fully utilize the inner volume, so the battery will have even higher energy density.
It is still another object of the present invention to provide reliable connection means of electrodes to the casings, to have stable low impedance, so the battery is high performance.
It is yet another object of the present invention to provide the right material and design of the cathode current collector and the cathode casing, to make them free from corrosion or oxidation during cycles, and provide long cycle life for such lithium-ion button cell battery.